I think that if you are looking for an image, then it will be better to re-post in the Photo Request forum.
Technically, I don't think you'll find an image which is that large at 300dpi. Most large format print work is produced at a lower resolution, hence there is no need to use 300dpi originals. For example, the artwork for most exhibition stands are produced at quarter size at 300dpi, so the effective DPI once the items are actually printed is only 75dpi.
This is more than adequate for these purposes, and means that you only need the original image to be a quarter of the finished item size.
Required print resolution in large format is determined mostly by viewing distance (well, this applies to any size but hand-held publications of course have a standard viewing distance - arms reach).
The larger the viewing distance, the lower the required print resolution. So as Stuart said, 75 - 100 dpi is pretty standard. 100dpi is actually pretty high for most large format. To find out exactly though, you're going to have to contact your printer and give him the size of your desired print as well as the situation it will be placed in, and approximate viewing distance expected.
As far as "maximum resolution"... I think you should be more concerned about the minimum resolution at that size. It's doubtful you'll be able to find an image of those dimensions with so much resolution that it will reduce the sharpness of the image (yes, you can get to that point with overtly high resolution). Even if you could find such an image, you and your printer would need some awful powerful computer equipment to handle it! So yeah... don't worry about the maximum, and just find out your minimum requirement from your printer, according to print dimensions and viewing distance.